1. Field of the Invention
This invention relates to an image-forming apparatus, in which an electrostatic latent image is formed by such means as an electro-photographic system or electrostatic recording system and developed with a developer including a toner and a carrier.
2. Related Background Art
In a well-known extensively used image-forming apparatus including a step of visualizing a latent image formed on an image carrier surface with a toner attached to the image, a two-component developer is used, which includes toner particles and carrier particles.
In the image-forming apparatus used with such a two-component developer, it is the toner that is actually consumed to form a developed image, and the carrier is hardly consumed. Therefore, when the developing operation is continued in this apparatus, the amount of toner contained in the developer is inevitably reduced to reduce image concentration. Therefore, it is necessary to detect the proportion of toner in the developer, i.e., toner concentration, and replenish with toner when the toner concentration is reduced.
Various toner concentration sensors have been proposed and used in practice. Among these means are optical means, which detects a change in the light reflectivity of the developer caused by a change in the toner concentration, and magnetic means, which detects a change in magnetic permeability of the developer.
However, the amount of toner that is consumed varies depending on the image ratio of an original that is copied; for instance, a difference in the toner consumption by ten or more times is produced between an original with an image ratio of 6% and an original with an image ratio of 100%. Therefore, if the amount of toner supplied from a toner replenishment port to a toner vessel or container in one cycle of replenishment is reduced, the toner concentration becomes insufficient eventually by continuing the copying of an original having a high image ratio.
On the other hand, when the amount of replenishment toner supplied per cycle is increased, a temporary excessive toner concentration condition is produced when replenishment toner is supplied while copying an original having a low image ratio, and this leads to deterioration of the image quality.
To solve the above problems, a small quantity of replenishment toner is supplied per cycle to reduce the change in the toner concentration at the time of supply of replenishment toner. When the copying of a high image ratio original is continued, the number of toner replenishment cycles is temporarily increased, or the copying operation is temporarily suspended and resumed after restoration of the toner concentration to a predetermined value.
The graphs of FIGS. 1 and 2 show the toner concentration controlled in the above way in the case when a toner concentration sensor provides a potential signal as a detected toner concentration signal. FIG. 1 shows the toner concentration when low image ratio original is copied, and FIG. 2 shows when a high image ratio original is copied. In the graphs, the ordinate is the sensor output voltage (representing toner concentration), and the ordinate is the number of copy sheets or time. In these Figures, designated at Vb is the voltage corresponding to the detected toner concentration, and Vc a voltage corresponding to a desired toner concentration To, i.e., reference value. The toner concentration of the developer can be controlled in the neighborhood of the reference value Vc by repeatedly causing operations of discontinuing replenishment toner supply when copying low image ratio original, i.e., when Vb.gtoreq.Vc, and resuming toner supply when Vb&lt;Vc. In other words, while replenishment with toner is being made according to toner consumption, the sensor output voltage Vb is varied, i.e., increased and reduced with respect to the reference value Vc, between values Vmax and Vmin corresponding to respective limit toner concentrations, i.e., limits of a permissible image density range.
However, when the image ratio is increased or when toner consumption exceeds toner replenishment so that a condition Vb&gt;Vc is continued for more than t seconds, for instance, a predetermined toner concentration can not be maintained unless the copying operation is temporarily suspended or the cycles of toner replenishment are increased. Where such toner concentration control is made, however, if the preset time t is too short, the operation noted above is caused even when the toner consumption does not exceed the toner replenishment. For this reason, a considerably long time has to be set as time t. Therefore, an insufficient toner concentration condition as shown at a in FIG. 2 or excessive toner concentration condition as shown at b due to temporary excessive toner replenishment are produced, leading to image quality deterioration.